PS plates comprising a hydrophilic support having thereon an oleophilic photosensitive resin layer have conventionally been widely used as negative working photosensitive lithographic printing plates. In general, a desired printing plate has been obtained from the PS plate by subjecting it to exposure to light (areal exposure) through a mask, i.e., a lith film, and then removing the nonimage areas of the resin layer by dissolution.
In recent years, the digitization technology in which image information is electronically processed, stored, and outputted with a computer is spreading widely. With this trend, various new techniques for image output system which are suitable for the digitization technology have come to be practically used. As a result, it is desired to establish a computer-to-plate (CTP) technology in which a photosensitive resin layer is scanned with a highly directional light such as a laser light according to digitized image information to thereby directly produce a printing plate without via the use of a lith film. An important technical subject therefore is to obtain a negative working photosensitive lithographic printing plate suitable for use in that technology.
A negative working photosensitive lithographic printing plate capable of such scanning exposure has been proposed which comprises a hydrophilic support having thereon an oleophilic photosensitive resin layer (hereinafter referred to also as “photosensitive layer”) containing a photosensitive compound capable of generating an active species, such as radicals or a Brφnstead acid, upon exposure to a laser light, and has already been put on the market. From this negative working photosensitive lithographic printing plate, can be obtained a negative lithographic printing plate by scanning it with a laser light based on digital information to generate an active species, causing the photosensitive layer to undergo a physical or chemical change by the action of the active species to thereby insolubilize the exposed areas of the layer, and subsequently developing the layer.
In particular, a negative working photosensitive lithographic printing plate which comprises a hydrophilic support having thereon a photopolymerization type photosensitive layer comprising a photopolymerization initiator having a high photosensitization speed, an ethylenically unsaturated compound capable of undergoing addition polymerization, and a polymeric binder soluble in an alkaline developing solution and which optionally has a protective layer having oxygen barrier properties attains excellent productivity, can be developed easily, and can give a printing plate having desirable printing performances because of its advantages of high resolution and satisfactory ink receptivity.
Organic polymers developable with an alkali have conventionally been used as polymeric binders serving as components of photosensitive layers. Examples thereof include the methacrylic acid copolymers, acrylic acid copolymers, itaconic acid copolymers, crotonic acid copolymers, maleic acid copolymers, and partially esterified maleic acid copolymers shown in JP-A-59-44615 (the term “JP-A” as used herein means as “unexamined published Japanese patent application”), JP-B-54-34327 (the term “JP-B” as used herein means an “examined Japanese patent publication”), JP-B-58-12577, JP-B-54-25957, JP-A-54-92723, JP-A-59-53836, JP-A-59-71048, etc.
However, the negative working photosensitive lithographic printing plates having a photosensitive layer containing such a conventional polymeric binder have had the following problem. When the speed of scanning is increased in order to further heighten productivity, the exposed areas come not to be sufficiently cured because the exposure light energy applied thereto per unit area becomes lower accordingly. As a result, the image areas are damaged during development by the alkali ingredient contained in the developing solution, so that high printing durability (i.e., high press life) cannot be obtained. Consequently, a further improvement in productivity has been difficult.
In JP-A-2000-89460 is disclosed a positive type photosensitive lithographic printing plate containing a vinyl copolymer having structural units comprising a cyclohexane ring and a carboxyl group. However, this technique is clearly different from the present invention.